Perspectives on fundamental cosmology from Low Earth Orbit and the Moon

www.nature.com/npjmgrav PERSPECTIVE OPEN Perspectives on fundamental cosmology from Low Earth Orbit and the Moon Gianfranco Bertone1 ✉, Oliver L. Buchmueller2 ✉ and Philippa S. Cole1 ✉ The next generation of space-based experiments will go hunting for answers to cosmology’s key open questions which revolve around inflation, dark matter and dark energy. Low earth orbit and lunar missions within the European Space Agency’s Human and Robotic Exploration programme can push our knowledge forward in all of these three fields. A radio interferometer on the Moon, a cold atom interferometer in low earth orbit and a gravitational wave interferometer on the Moon are highlighted as the most fruitful missions to plan and execute in the mid-term. 1234567890():,; npj Microgravity (2023)9:10 ; https://doi.org/10.1038/s41526-022-00243-2 INTRODUCTION The standard cosmological model provides a simple framework to explain a variety of observations, ranging from sub-galactic scales to the size of the observable universe. Yet many open questions remain: the model relies on an unknown mechanism for the production of perturbations in the early universe, on an unknown matter component, generically referred to as dark matter, and on an unknown mechanism that leads to an accelerated expansion of the universe, generically referred to as dark energy. The next generation of space-based experiments are our best chance of unveiling these mysteries. A united front of low earth orbit and lunar missions, as outlined in the European Space Agency’s (ESA) Human and Robotic Exploration (HRE)1, will break unprecedented ground on all of these fronts. Alongside the Laser Interferometer Space Antenna2, a radio interferometer on the Moon, a cold atom interferometer in low earth orbit and a gravitational wave interferometer on the Moon would provide a full-coverage approach to unravelling the key open questions in cosmology today. In section “Key knowledge gaps” the key knowledge gaps in cosmology are highlighted, in section “Priorities for the space programme” specific suggestions for experiments that should be the priorities for ESA’s space programme and which questions they will answer are laid out, before concluding and discussing the future outlook in section “Future outlook and summary”. KEY KNOWLEDGE GAPS Inflation The theory of inflation is arguably the most promising model of the physics of the early universe3. The paradigm postulates that quantum fluctuations went on to seed the cosmological perturbations that we see imprinted on the Cosmic Microwave Background (CMB) and were the beginnings of all of the structure in the universe today. And yet, much remains to be understood about the properties of the quantum field that supposedly led to the initial period of exponential expansion of the universe. Whilst the paradigm is fully consistent with cosmological data4–6, we still currently lack direct smoking-gun evidence supporting it, as well as a specific model for how one or more scalar fields drove the expansion. On large scales, k ~ 10−3 − 0.1 Mpc−1, observations of the CMB temperature anisotropies by Planck6 have confirmed to incredible precision that density perturbations were small (fluctuations of order 10−5) and almost scale-invariant. The simplest single-field, slow-roll models of inflation are able to describe this spectrum of the density perturbations. However, deviations from scaleinvariance on small scales could indicate a more complicated model that exhibits a feature in the inflationary potential. Such models could have interesting observational signatures, such as ultra-compact mini-haloes7,8 or primordial black holes9. Furthermore, primordial non-Gaussianity has been constrained to be small, fNL,local = − 0.9 ± 5.1, on large scales10. This constraint has limited the viability of many models of inflation that predicted larger values of primordial non-Gaussianity, for example DBI inflation and EFT inflation11,12. However, reaching the fNL,local < 1 threshold will provide strong evidence that observations are not consistent with multi-field models of inflation13. The final piece of the puzzle can be provided by the tensor-to-scalar ratio, which is currently constrained to be less than 0.16, a measurement of which would indicate the energy scale at which inflation happened. Dark matter Similarly, the existence of dark matter is supported by a wide array of independent observations, but we still know very little about the fundamental nature of this elusive component of the universe. In the past four decades, a strong effort went into the search for a particular class of candidates: weakly interacting massive particles14. However, no experiment has yet found evidence for these particles, and attention has turned to different classes of dark matter candidates in regions of parameter space where they would have evaded strong constraints from direct detection before now, for example axion-like-particles (ALPs)15–17 or primordial black holes (PBHs)18. Axion-like-particles are in particular a popular dark matter candidate15–17. The QCD (quantum chromodynamics) axion was first postulated in the 70s to solve the strong CP problem19. 1 Gravitation Astroparticle Physics Amsterdam (GRAPPA), Institute for Theoretical Physics Amsterdam and Delta Institute for Theoretical Physics, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands. 2Imperial College London, Exhibition Rd, South Kensington, London SW7 2BX, United Kingdom. ✉email: ; ; Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA G. Bertone et al. 1234567890():,; 2 However, ALPs more generally, often motivated by string theories in which ultra-light particles are ubiquitous, display the qualities required to explain all or part of the dark matter. Whilst searches for the standard axion with a mass of order of a few hundred keV have yielded no detections, "invisible” axions with very small masses are still viable candidates. Search strategies vary depending on the mass of the axion, which can’t be theoretically predicted, but the most common approach is to probe their interactions with electromagnetic fields and constrain the axionphoton coupling20. Astrophysical observations are able to look for signatures of axion to photon conversion in the presence of electromagnetic fields, for example, by looking for such processes in the vicinity of the magnetosphere of neutron stars21–23, or their production in the solar core, triggered by X-rays scattering off electrons and protons in the presence of the Sun’s strong magnetic fields24. For masses less than 1eV, axions are a sub-set of the broader class of ultra-light dark matter models, with masses down to (theoretically) 10−24 eV, although Lyman-alpha forest constraints have ruled out axion masses less than 2 × 10−20 eV25, see26 for a review. Ultra-light dark matter models postulate a new ultra-light boson, which displays wave-like propert (...truncated)